Summer is smoke and dust season, so it's time to look for the crepuscular rays formed when sunlight illuminates tiny particles in the sky.

Crepuscular rays are shafts of sunlight cast across the sky through gaps in clouds or even mountains. They are difficult to see when the sky is clear. But they become very obvious when the sky contains dust, smoke, fog or other particles known as aerosols.

Crepuscular rays can be colorful and even beautiful. They add a sense of dimension to the sky.

Between crepuscular rays are dark rays formed by the shadow of the cloud or mountain through which rays of sunlight are projected.

The bright rays and adjacent shadows are as straight as arrows.

While they appear to radiate out from the sun, this is an optical illusion caused by the perspective of the viewer. The rays are actually parallel.

They appear to converge just as parallel railroad tracks and divided highways seem to merge together in the distance.

Besides providing a visual treat, the seasons when crepuscular rays appear provide clues about the aerosols that formed them.

If you observe crepuscular rays during spring, they might have been formed by dust from Asia or smoke from Mexico. Those that appear during summer can be caused by African dust, Asian smoke or air pollution from states northeast of Texas.

Crepuscular rays also allow you to see the distribution in the sky of the aerosols that formed them.

Often the brightness of a ray will appear unchanged as it approaches the ground. This suggests a very well-mixed atmosphere.

I photograph crepuscular rays to keep a record of their appearance. My goal is to develop a website that shows atmospheric measurements and a wide range of sky photographs going back to 1990.

The New Mexico state flag is based on a symbol that appears to show crepuscular rays. The flag features a red circle symbolizing the sun on a yellow background.

Four sets of four red rays emerge from each of the four cardinal directions around the sun.

The sun design on the New Mexico flag is known as the Zia symbol, since its origin has been traced to a similar design on a pot from Zia Pueblo near Albuquerque.

The people of Zia Pueblo assign a religious role to their symbol, but outsiders have used it to decorate everything from license plates to advertisements.

You can learn more about the zia symbol and the pueblo's reaction to its widespread use from an article by Wendy Brown in the New Mexican newspaper.